FIG. 1 is a typical dry mill process having a back-end oil recovery system. FIG. 2 is a typical dry mill process with a back-end oil and protein recovery system. FIG. 3 is a dry mill process having a front-end dewatering mill system and a front-end oil recovery system. FIG. 3A is a dry mill process having a front-end dewatering mill system, a front-end oil recovery system, and a back-end protein recovery system.
The typical methods of producing alcohols from grains generally follow similar procedures depending on whether the process is operated under wet or dry conditions. Wet mill corn processing plants convert corn grains into several different co-products, such as germ (for oil extraction), gluten feed (high fiber animal feed), gluten meal (high protein animal feed), and starch-based products, such as ethanol, high fructose corn syrup, food, and industrial starch. Dry grind ethanol plants convert corn into two products, including ethanol and distiller's grains with soluble. If it is sold as wet animal feed, distiller's wet grains with soluble is referred to as DWGS. If it is dried for animal feed, distiller's dried grains with soluble is referred to as DDGS. In the standard dry grind ethanol process, one bushel of corn yields approximately 8.2 kg (approximately 17 lbs.) of DDGS in addition to approximately 10.3 liters (approximately 2.75 gal) of ethanol. These co-products provide a critical secondary revenue stream that offsets a portion of the overall ethanol production costs. DDGS is normally sold as a low value animal feed even though that the DDGS contains 11% oil and 33% protein. Some plants start to modify the typical process to separate the valuable oil and proteins from the DDGS. Currently, some plants recover oil from syrup by one stage centrifuge, such as a decanter or a disc centrifuge.
Because the costs of wet grinding mills are so prohibitive, some alcohol plants prefer to use a simpler dry grinding process. FIG. 1 is a flow diagram of a typical dry grind ethanol producing process 10. As a general reference point, the dry grind ethanol process 10 is divided into a front-end process and a back-end process. The part of the process 10 that occurs prior to a step of distilling 14/fermenting 13 is considered as the “front-end” process and the part that occurs after the step of distilling 14/fermenting 13 is considered as the “back-end” process. The front-end process of the process 10 begins with a grinding step 11 in which dried whole corn kernels are passed through a hammer mill to be ground into corn meal or a fine powder. The screen openings in the hammer mills are typically of size #7 or about 2.78 mm, which result in a wide spread of particle sizes distribution. The particle sizes mentioned above are as small as 45 micron and as large as 2 to 3 mm. The ground meal at the grinding step 11 is mixed with water to create slurry. A commercial enzyme called alpha-amylase is added (not shown) to the slurry.
In a liquefying step 12, the slurry is heated to approximately 120° C. for about 0.5 to 3 minutes in a pressurized jet cooking process to gelatinize (solubilize) the starch in the ground meal. In some typical processes, a jet cooker is not used and a longer tank holding time is used instead. The pH value at the liquefying step 12 is adjusted to about 5.8 to 6 and the temperature is maintained between 50° C. to 105° C. to convert the insoluble starch in the slurry to become a soluble starch. The stream after the liquefying step 12 has about 30% dry solids (DS) and all other components that are in the corn kernels, including sugars, protein, fiber, starch, germ, grit, oil and salt. There are generally three types of solid particle sizes larger than 50 micron in the liquefying stream: fiber, germ, and grit, which have about the same particle size distributions in all three types of solids.
The liquefying step 12 is followed by a simultaneous saccharifying and fermenting step 13. This simultaneous step 13 is referred in the industry as “Simultaneous Saccharification and Fermentation” (SSF). In some commercial dry grinding ethanol processes, saccharification and fermentation occur separately (not shown). Both individual saccharification and SSF take as long as about 50 to 60 hours. Fermentation converts the sugar to alcohol using a fermenter. Subsequent to the saccharifying and fermenting step 13 is a distilling (and dehydrating) step 14, which utilizes a still to recover alcohol.
Next to the distilling step 14 is a fiber separating step 15. The fiber separating step 15 centrifuging the “whole stillage” produced at the distilling step 14 to separate the insoluble solids (“wet cake”) from the liquid (“thin stillage”). The “wet cake” includes fibers (per cap, tip cap, and fine fibers), grits, germ particles and some proteins. The liquid from the centrifuge contains about 6% to 8% of DS, which contains mainly oil, germs, fine fibers, fine grits, protein, and soluble solids from the fermenter and ash from corns. The whole stillage in some plants with an average of about 14% of DS is fed to the first stage evaporator that is concentrated to 16˜25% DS before feeding to the fiber separating step 15.
The thin stillage from the fiber separating step 15 is divided into two streams. The first stream containing about 30 to 40% of the flow is recycled back (as a “back-set” stream) to be mixed with corn flour in a slurry tank at the beginning of the liquefying step 12. The second stream that contains the rest of the flow (about 60 to 70% of the total flow) enters evaporators in an evaporating step 17 to boil away some water moisture leaving oil, protein (gluten and yeast), and a thick syrup that contains mainly soluble (dissolved) solids in the corn. The back-set water is used as part of cooking water in the liquefying step 12 to cut the fresh water consumption as well as save evaporating energy and equipment costs.
The concentrated slurry is able to be subjected to an optional oil recovering step 16, whereat the slurry can be centrifuged to separate oil out from the syrup. The oil is able to be sold as a separate high value product. The oil yield is normally about 0.4 lbs./Bu of corn with a content of high free fatty acids (around 15% FFA). This back-end oil recovering system recovers only about ¼ of the oil in the corn. About one-half of the oil inside the corn kernel remains inside the germ after the fermenting/distilling step 13/14. Only about 1 lb./Bu of free oil is available to be recovered from the syrup and the other half (about 1 lb./Bu oil) still remains inside the germ oil cell. Most of the not recovered oil (inside the germ oil cell) goes out with decanter cake (DDG).
About 0.5 lb./Bu of the oil in the syrup (thin stillage) is trapped/absorbed in the fine fibers or forms an emulsion layer (having a density around 1 g/ml) with proteins, which cannot be separated in a typical dry grind process using one stage centrifuges. The emulsion layer stays and builds up inside the machine, which affects the oil separation inside the centrifuge. There are ways to break emulsion (decrease amount of emulsion) and increase the oil yield by adding chemicals (e.g., emulsion breaking chemicals and emulsion breakers), adding alcohol to an extraction step, or heating to a higher temperature. Although the above three methods are effective in reducing emulsions, each of the methods has its drawbacks. Although adding chemicals, such as emulsion breaker, is able to improve the separation efficiency in some degrees, chemicals are costly and the DDGS product can be contaminated by the added chemicals. Providing heat or raising the feed temperature at the centrifuge to break the emulsion is another way to improve the separation efficiency, but the temperature affects the quality of oil and DDGS (mainly higher free fatty acid and darker color). Adding an alcohol to break the emulsion also improves the separation and increases the oil yield, but it needs exploration proof equipment and costly operations. All these improvements only increase the oil yield from an average of 0.4 lbs./Bu to about average 0.6 lbs./Bu. About 0.4 lbs/Bu of the oil trapped/absorbed is still not able to be recovered. The oil/protein emulsion formed during the whole dry mill process is the main reason having a low oil yield in the back-end oil recovery system.
An oil and protein recovery process (PCT/US09/45163, filed on May 26, 2009; titled “METHODS FOR PRODUCING A HIGH PROTEIN CORN MEAL FROM A WHOLE STILLAGE BYPRODUCT AND SYSTEM THEREFORE,” which is incorporate by reference in its entirety for all purposes), is developed by adding an oil/protein separating step to break this oil/protein emulsion in the thin stillage. As shown in the process 20 of FIG. 2, the front-end process is the same as the existing dry mill process described in the FIG. 1. The process changes its procedures after the fiber separating step 15, which is part of the back-end process. The oil/protein separating step 21 is added between the fiber separating step 15 and the evaporating step 17. A high G force centrifuge, such as nozzle centrifuges and disc decanter, are able to be used at this step instead of other types of disc centrifuges or lower G force decanter, because the high G force nozzle is more effective in breaking the oil/protein emulsion. The thin stillage from the fiber separating step 15 is fed to the oil/protein separating centrifuge step 21. The oil/protein emulsion is broken up by the high G force inside the centrifuge. The overflow discharge (light phase) that contains the free oil is discharged from the top of nozzle centrifuge with a portion of a liquid as an overflow. The underflow discharge (heavy phase) that contains protein and fine fiber, which is heavier than the liquid, is discharge from the nozzle with a portion of the liquid in slurry. The oil/protein emulsion is broken up under a high G force inside the disc stock. The oil that is trapped and/or absorbed by the fine fiber is able to be released by using the density difference of the oil (0.9 gram/ml), protein (1.05 gram/ml), and fiber (1.1 gram/ml). The overflow is then fed to an evaporator step 17 to be concentrated to have a 25 to 40% of DS (forming a semi-concentrated syrup).
Next, the semi-concentrated syrup is sent to the back-end oil recovery system step 16 for oil recovery. The overflow stream from oil/protein separating step 21 contains less protein, so it has less chance to form oil/protein emulsion during the evaporator stage. The oil yield with this system reaches 0.9 lb./Bu. The de-oil syrup from the back-end oil recovery step 16 is able to be further concentrated in an evaporator to have a higher syrup concentration with as high as 75% of DS. The de-oil syrup with a low protein content can avoid fouling at the evaporator. The underflow from oil/protein separating step 21 goes to a protein dewatering step 22 for protein recovery. The separated protein cake from protein dewatering step 22 having a content of less than 3% oil is sent to a protein dryer at a protein drying step 23 to produce a high value protein meal, which has a protein content of 50%. The liquid from the protein dewatering step 22 is sent back to the front-end as a back-set liquid that is used as part of cooking water in the liquefying step 12.
The process described in the patent application (PCT/US12/30337; file on May 23, 2012; titled “DRY GRIND ETHANOL PRODUCTION PROCESS AND SYSTEM WITH FRONT END MILLING METHOD” is incorporate by reference in its entirety for all purposes. The process having a front dewatering mill and a front oil recovery system is shown in a process 30 of FIG. 3. In the process 30, a dewatering mill system is added to a liquefying step 32. The step 32 is followed by front-end oil recovery steps 31 and 33, such that oil is able to be recovered in the front-end (i.e., before the fermenting step 13). A three phase nozzle centrifuge at step 31 is used to separate the content into a light phase, a heavy phase, and a nozzle phase. The heavy phase contains protein. The light phase contains oil, emulation, and germs, which are from the liquefied starch solution. The light phase is subsequently sent to an oil polish centrifuge step 33 to recover pure oil. The oil that is recovered at the front-end has a much lighter color, lower fatty acid, and is able to be more easily separated in the centrifuge, because a higher liquid density and less oil/protein formation is resulted by using this process. However, the oil yields only increased to 0.4 lb./Bu.
About ¼ of the oil (about 0.5 lb./Bu) in the germ is released during the fermenting step 13 and distilling step 14. A back-end oil recovering step 16 is used to recover more oil that is not released before the fermenting step 13 and distilling step 14. This combined front-oil recovering step 16 and back-end oil recovering step 31 can generate 0.4 lb./Bu oil at the front-end and 0.3 to 0.6 lb./Bu oil in the back-end depending on the type of emulsion breaking step used.
As shown in FIG. 3A, the protein recovering system is added to the system of FIG. 3. The system in FIG. 3A removes, recovers, and produces protein meal and increases the back-end oil yield by avoiding the formation of oil/protein emulsion during the evaporation process, which is also described in the U.S. Provisional Patent Application Ser. No. 61/692,593, filed Aug. 23, 2012 and entitled “A SYSTEM FOR AND METHOD OF SEPARATING OIL AND PROTEIN FROM GRAINS USED FOR ALCOHOL PRODUCTION,” which is incorporated herein by reference in its entirety for all purposes. The oil/protein separating step 21, the protein dewatering step 22 and protein drying step 23 are added to the system of FIG. 3 to become the system of FIG. 3A. The protein drying step 23 (very high equipment cost) can be eliminated if producing protein meal is not needed. The protein wet cake from the protein dewatering step 22 is mixed with syrup from the evaporating step 17 and DDG cake from the fiber separating step 15 to produce DDGS by-product. The oil in the back-end increases to 0.5 to 0.8 lb./Bu depending on the number of dewater milling steps in the front-end.
Still referring to FIG. 3A, a two phases (Clarifier) nozzle centrifuge is used to effectively break the bonds between oil and protein and release the oil that is trapped/absorbed in the fine fiber. The oil (lighter than liquid) is discharged with the liquid in a slurry form as overflow discharge. The protein and fine fiber (heavy than liquid) are discharged with the liquid in slurry form as nozzle flow discharge (underflow). The overflow discharge contains free oil, oil/protein emulsion, and germ particles, which is used as a back-set stream to recycle back to the front-end. Accordingly, the oily stream (contained in the overflow) in the back-end process is able to going back to the front-end with the back-set stream to be recovered by the front end oil recovering step 31. In the typical dry mill process, two separate oil recovery systems (one in the front-end step 31 and one in the back-end step 16) are needed.